The Internet is a loose network of connected computers spread throughout the world. A message can be sent from any computer on the Internet to any other by specifying a destination address and passing the message from computer to computer via a series of "hops." Each computer, router, or "node" on the Internet has a unique Internet address. When an intermediate computer or router receives a message in transit, the computer checks the intended destination of the message and passes it along accordingly.
The Internet is growing, in terms of both size and sophistication, at a rapid rate. In the past, most users of the Internet were academic, research, or institutional users; the Internet was primarily used at that time to transmit and receive electronic mail and network news and to allow transfer of computer files. However, since the introduction of the World Wide Web (also known as the "Web" or the "WWW") several years ago, the Internet has begun to host increasing amounts of other types of data of general interest, namely representations of images, articles, etc.
The Web protocol and language establish a graphical means to navigate the expanses of the Internet. "Web pages," often consisting primarily of text and graphical material, are stored on numerous computers, known as "Web servers," throughout the Internet. A software program known as a "browser" can be used to access and view Web pages across the Internet by specifying the location (i.e. Internet address) of the desired Web page. When a Web page is accessed, its information is transmitted from the remote computer (server or delivery site), wherever in the world it may be located, across the Internet, to the user.
In recent times, the Web has begun to host highly sophisticated types of multimedia content, such as audio and video data, and computer software. Compared to first generation Web content, namely text and still images, audio clips, video clips, and software programs have extremely high storage and bandwidth requirements.
At present, it is difficult, if not impossible, to provide sustained high-speed transmission of large audio/video files over a multi-node link on the Internet. Because the data is often transferred from afar, many factors can cause the delay or even loss of parts or all of a transmission. It is generally not critical if a user experiences minor delays in receiving small graphic or text files. However, it is recognized that real-time data such as video has very specific and stringent timing requirements for data transfer and display.
Unfortunately, the present design of traditional Internet-like data networks is based on the principle that delays and significant data transmission rate variations are acceptable for ordinary data (e.g. text and still images). Consequently, because of the high value of permitting access to text and graphical information from locations around the world, such transmission defects are considered acceptable, and the base capacity of the Internet is somewhat "oversubscribed" to reduce data transmission costs. In other words, the timeliness of network data transmission has been significantly compromised in order to render relatively insignificant the aggregate cost of long distance communication connections.
In order to successfully transfer audio-video data across a message-oriented network such as the Internet, for any more than a few users, network resources should be committed in a manner facilitating timeliness of transmittal. A system using committed network resources generally cannot take advantage of the existing pricing scheme of shared networks like the Internet, since it cannot participate in the sharing of network resources on a data packet by data packet basis. Video data must be transmitted to the exclusion of lower-priority data. Transmission costs thus become significant, especially when the connection is "long distance" or when the connection is continued over an extended period of time.
Another consequence of the timeliness vs. cost compromise discussed above has been the seemingly indiscriminate topographical design of the network. Since delays and throughput variations have traditionally been excused in favor of low cost, the configuration of the Internet infrastructure has also been driven by cost considerations. Accordingly, the interconnection efficiency of the network has rarely been considered. The rapid growth of real time data is changing this requirement.
It is recognized that inadequate data transfer performance of time-sensitive data on the Internet is typically caused by four factors: packet loss, excessive server utilization, the relatively low capacity of the network infrastructure, and inherent delays in the network hardware. Packet loss, in particular, is caused by inadequate infrastructure and lack of robustness in routing. The inherent delays are believed to be caused by, among other things, the lack of flow control between adjacent nodes in a multiple-node path on the Internet.
Unlike smaller text and graphic files, relatively large video files can take several minutes (or more) of "streaming," or constant data flow. Consequently, the usual network performance problems are exacerbated. Network bandwidth, or the data-carrying capacity of a particular network, is limited. Thus, packet loss and delays increase. Long delivery times consume a large amount of server capacity for a long time, decreasing the resources available to other users. Accordingly, because the network infrastructure becomes increasingly congested, packet loss and delays continue to increase, transmission times rise, and server load increases further.
This pattern exemplifies a "downward spiral" of network performance, which can be driven by the attempted transmission of large data files such as video clips. As long as network traffic remains within the limits imposed by network bandwidth, network performance will remain acceptable. However, whenever peak network loads exceed capacity, the downward spiral described above will begin, causing increasing periods of poor network performance.
As discussed above, a browser program can be used to access and view Web pages across the Internet by specifying the location (i.e. Internet address) of the desired Web page, or more commonly, by "hotlinking" to Web pages. Common browsers are Lynx, NCSA Mosaic, Netscape Navigator, and Microsoft Internet Explorer. The desired Web page is specified by a uniform resource locator ("URL"), indicating the precise location of the file using the syntax "http://internet.address/directory/filename.html".
Web pages are generally described, in terms of layout and content, by way of a language known as "HTML" (HyperText Markup Language). Any particular computer linked to the Internet can store one or more Web pages, i.e. computer files in HTML format, for access by users.
Hotlinking from one HTML Web page to another is accomplished as follows. The user first accesses a Web page having a known address, often on the computer located at the user's ISP (Internet Service Provider). The ISP is the organization providing Internet connectivity to the user. That Web page can contain, in addition to textual and visual data specified in HTML format, "links," or embedded information (in the form of URLs) pointing to the Internet addresses of other Web pages, often on other computers throughout the Internet. The user, by selecting a link (often by pointing and clicking with a mouse), can then access other Web pages, which can in turn contain further data and/or additional links.
Various extensions to HTML, such as Netscape's EMBED tag, allow references to other data to be embedded into Web pages. Some browsers are not capable of handling data other than text and images. Other browsers can handle the data in various ways. NCSA Mosaic, for example, handles references to unknown types of data by allowing the data to be downloaded to the user's computer, and then optionally invoking an external program to view or manipulate the data. Recent releases of Netscape Navigator and Microsoft Internet Explorer take the concept one step further: a browser extension, or "plug-in," can be automatically invoked to handle the data as it is received from the remote Web page. Other means, such as network program "applets" written in the Java language (or a similar language), can be used to extend the functionality of the browser environment or network.
Digital multimedia data can have extremely high storage and bandwidth requirements. In particular, video files can be very large, from approximately 10 megabytes to 10 gigabytes. In order to play video files at speeds approaching their recorded rate at a user's terminal, the files have to be delivered at a fast, constant speed. Too slow, and the image plays back slower than originally recorded. If the speed is uneven, then the video appears jerky, like an old-time movie.
The network design compromises discussed above generally adversely impact the transmission of audio and video data across the Internet. While a user using a browser to "surf" the Web might not notice minor delays and transmission rate variations while retrieving text and still images, such defects become apparent and significant when real-time audio and video information is accessed.
In an attempt to solve these problems, Internet content providers sometimes spread popular content around the Internet on various servers or delivery sites known as "mirror sites." Each mirror site contains information that is essentially identical to that of the original site. For example, if a popular Web site is located in New York, mirror sites might be located in Los Angeles, London, and Tokyo. Accordingly, if a European user is having difficulty accessing the original New York site, he can hotlink to the mirror site that is geographically closest, i.e. London.
However, mirror sites have several disadvantages. For example, mirror sites may be widely distributed geographically, but may not be efficiently distributed on the network in terms of actual usage, network traffic, etc. Thus, New York and Los Angeles mirror sites might both be connected to the same national Internet service provider's network, meaning that difficulty in accessing one of the sites might also affect the other.
Furthermore, mirror sites might not be optimally placed to reduce load on each server. Although an "educated guess" might be made as to where a mirror site should be located, actual usage patterns might differ. Furthermore, there is no guarantee of enhanced performance. The bandwidth of the mirror site might be lower than that of the original site, or it might be overloaded for other reasons.
Moreover, mirror sites are often hosted on a voluntary basis. If a Web site is extremely popular, and a service provider determines that the subject matter might be of interest to its subscribers, that service provider might agree to host a mirror site of the original Web site. Such an arrangement would be attractive to host of the mirror site because people would be drawn to the mirror site, and might hotlink to other content hosted there. On the other hand, such voluntary alliances typically are not reliable and might be severed at any time.
In essence, a mirror site offers a secondary source for data, which may or may not be available, and which may improve user convenience, but which does not address network bandwidth or efficiency. A mirror site does not account for performance characteristics of the network, nor identify available bandwidth which could be used to efficiently transmit video data while still taking advantage of the existing low-cost pricing schemes such as those on the Internet.
Currently, there is no guidance in selecting optimal locations for delivery sites, nor is there a known method permitting a user to determine which mirror site to connect to that will ensure optimum performance. In fact, the use of a traditional mirror site is voluntary. Typically, a user will try to access the original site (or a known mirror site), and will switch to another mirror site only if performance is found to be insufficient after one or more attempts. This approach is an inefficient utilization of network resources. Clearly, mirror sites are not an optimum solution to the problem of overloaded Web sites. A principal reason for this, among others, is the failure to consider network performance.
Network analysis, particularly the performance of specific paths and links over the Internet, is well known and developed. For example, the "ping" program allows a computer connected to the Internet to determine whether a remote host is accessible. However, the ping program uses a low-priority network protocol known as the ICMP protocol, and accordingly does not provide meaningful performance analysis information. The "traceroute" program follows the transmission of a message from a computer to a remote host, tracking delays along each link, and determining the path taken by the message. The traceroute application can be used to map the flow of data. However, it lacks the ability to provide meaningful performance analysis information. Traceroute only provides route information for a message propagating in one direction, and only for one instant in time.
Moreover, only the connectivity characteristics of paths leading to and from the single computer running the tests are typically determined; expanding the scope of testing is possible but logistically impracticable, since the Internet is so large.
Traditional network analysis techniques such as the "ping" and "traceroute" programs offer a view of network connectivity but provide little understanding of what performance can be expected from providers and mirror sites across the Internet. Therefore, only "guesses" can be made as to where delivery or mirror sites should be located or which mirror sites should be used to optimize performance.
Accordingly, a need exists for a method of determining overall network performance. A further need exists for a system applying that method to enable content providers to dynamically locate data delivery or mirror sites at optimum network locations, and to allow users to select optimum mirror sites from which to receive data.